Nsryjdtyk

A week ago I finally found a way to split up my messages in different categories (Here is the SO question where I got my answer)

Now I have implemented this solution:

type Msg 

    = AMsg AMsg

    | BMsg BMsg

    | CMsg CMsg

and then I defined AMsg as follows

type AMsg

    = ActionOne Int String

    | ActionTwo Int

Everything works great except when I use ActionOne with a onInput Html.Event

input [onInput (AMsg (ActionOne model.id))] 

This errors out telling me that onInput needs a String -> Msg type but is getting a AMsg type.

This would work just fine if I was using onClick for example and pass in the second argument myself

input [onClick (AMsg (ActionOne model.id "hello"))] 

But since I need to use onInput and that is what passes the second String parameter to ActionOne I am stuck. If I change my types to adapt to this

type Msg 

    = AMsg AMsg String

    | BMsg BMsg

    | CMsg CMsg



type AMsg

    = ActionOne Int

    | ActionTwo Int

This works, but then i force ActionTwo to also take a string, which i don't want.

Otherwise I am stuck specifying ActionOne directly under Msg

type Msg 

        = AMsg AMsg

        | BMsg BMsg

        | CMsg CMsg

        | ActionOne Int String



type AMsg

    = ActionTwo Int

I really care about separating Msgs into different categories -- if I want to use elm for a bigger project this seems pretty crucial to me. Any ideas ?

The function that you pass to onInput has to have the type String -> msg (or more specifically in your case, String -> Msg.)

You can accomplish that by using a lambda:

input [onInput (str -> (AMsg (ActionOne model.id str)))] 

You can also use function composition (the << function) to achieve the same thing without needing to talk about the parameter str explicitly:

input [ onInput <| AMsg << ActionOne model.id ]

To expand a bit on answer of Matt McHenry:

Sometimes when you have more complicated cases, it helps to think more closely what the usual simpler syntax actually means. When you have the basic case of

type Msg

    = GotInput String

...

input [onInput GotInput] 

then GotInput here is a function String -> Msg. You can also write this out explicitly as:

input [onInput (str -> GotInput str)] 

So here you give onInput a function which takes str parameter and then returns GotInput str.

Now that you have this basic function, it can become clearer how to extend this to more complex case - you just change return value and include str at correct place in it:

input [onInput (str -> (AMsg (ActionOne model.id str)))]